1. Field of the Invention
The invention relates to videoendoscopes equipped with a device for measuring the dimensions of a target located in the observation field of the videoendoscope.
It is particularly, but not exclusively, applicable to industrial endoscopy.
2. Description of the Prior Art
Normally, the term “endoscope” is given to a flexible or rigid probe, designed to be inserted into an obscure cavity and allowing the user to observe through an eyepiece the image of a target located inside the cavity. To this end, an endoscope comprises a lighting device of the target and an optical device providing the user with an image of the target. The optical device comprises a distal lens, an image transportation device that may be rigid and composed of a series of lenses, or flexible composed of a bundle of sorted optical fibers, and a proximal eyepiece in which the user can observe the image of the target. The lighting device normally comprises a bundle of lighting fibers whose distal tip, conveniently directed near the distal lens, lights the target when its proximal tip is connected to a light generator.
The term “videoendoscope” therefore refers to a flexible or rigid probe that allows the user to observe on a video screen the image of a target located inside an obscure cavity. To this end, a videoendoscope comprises a lighting device of the target that is identical to that of an endoscope and an imaging device that provides the user with a video image of the target. The imaging device comprises an optoelectronic device composed, in particular, of an optical device and a CCD (Charge Coupled Device) sensor on the sensitive surface from which the image is formed of the target delivered by said optical device, a video processor electrically connected to the CCD sensor that transforms the electric signal delivered by the CCD sensor into a video signal, a control panel for adjusting the main operating parameters of the video processor, and a video monitor for viewing the video signal delivered by the processor.
The term “videoendoscopic probe” refers to a videoendoscope comprising the following elements:                a distal end part that houses an optoelectronic imaging device including, notably, a lens and a CCD sensor,        a flexible inspection tube whose distal end is integrated into the distal end part,        an integrated control handle of the proximal end of the inspection tube,        a flexible umbilical connection cable distal end of which is integrated into the control handle and the proximal end of which is designed to be connected to an external unit that integrates, notably, a light generator and an electric power supply source,        a bundle of lighting fibers housed in the umbilical cable, in the control handle, then in the inspection tube and whose distal end, housed in the distal end part, lights the target when its proximal end is connected to a light generator,        a video processor electrically connected to the distal end part and whose synchronization is adjusted according to the length of the electric cable connecting it to the distal end part,        a video monitor connected to the video processor, and        a control panel that allows adjusting the operation of the video processor and possibly the video monitor.        
The recently designed videoendoscopic probes can include, among other things, the following elements:                an articulated distal tip deflection that allows modifying the direction of the distal end part of the probe, the control handle that then generally integrates the mechanical or electromechanical command means for activating this tip deflection,        interchangeable optical heads that can be adapted to the distal end part of the inspection tube and that allow modifying the optical field open by the videoendoscope and/or the directions of the optical view and light axes of the videoendoscope, and        an image freeze, recording, pointing, and processing digital system that can be directly controlled by the control panel of the videoendoscope. Such a system can, among other things, be used for metrology purposes.        
Within the scope of inspection of mechanical pieces, it might be desired to add to the viewing function of the videoendoscope a metrology function enabling the user to directly measure the dimensions of certain elements of the observed mechanical piece. The implementation in a videoendoscope of a metrology function is generally performed using the following means:                an optical means that can be integrated into the opto-electronic distal device of the videoendoscope for viewing a parameter whose position in the image delivered by the videoendoscope reflects the true distance separating the distal end of the videoendoscopic probe and the target to be measured,        an electronic means connected to the videoendoscope for enabling the operator to point on the video monitor the above-mentioned parameter, as well as the ends of the target to be measured, and for implementing a mathematical algorithm that can deduce from these points the observation distance and the dimensions of the observed target.        
The metrology methods that are most commonly used first in traditional endoscopy and then in videoendoscopy are briefly described below.
Direct Measure by Moving the View Axis
This method is only applicable to rigid side view endoscope and consists in pointing using optical means (U.S. Pat. No. 4,702,229) or electronic means (U.S. Pat. No. 4,820,043) the ends of a target by moving lengthwise the endoscope by one mechanical measurable value.
Measure by Optical Crosshairs
This method is also only applicable to rigid endoscopes and only for repetitive measurements performed on identical targets. It consists in optically superimposing on the image of the target an image of a graduated reticule placed on the distal focal plane of the endoscope eyepiece and specific to the mechanical piece forming the target. Endoscopes equipped with such a reticule are generally deviated view endoscopes that include attached devices for focusing, varying the view angle, rotating the view axis around the endoscope axis, and also rotating the reticule. Such endoscopes are described in patents U.S. Pat. No. 6,333,812 and FR 2 832 516. In document JP11045649, also foreseen is the electronically superimposing on the video image of the target a meshed network specific to the mechanical piece to be measured.
Measure by Moving an Optical Component
This method is applicable to rigid endoscopes the optical device proximal end of which may be integral with an opto-electronic device including notably a lens and a CCD sensor. It consists in visually adjusting an optical parameter by moving lengthwise one of the optical components of the endoscope optical device along a distance which can be directly measured using mechanical, optical or electronic means, and deriving the observation distance and then the dimensions of the target from the distance of this movement. The optical parameter to be adjusted can be the image sharpness that results from moving the proximal adjustment lens (U.S. Pat. No. 6,100,972, WO 96/20389) or the distal lens (U.S. Pat. No. 4,558,691) of a low field depth optical device. This parameter can also be the image magnification (U.S. Pat. No. 6,476,979) resulting from the movement of one of the lenses of an optical device with a variable focal length.
Measure by Comparison
This method can only be used in videoendoscopy and consists in subtracting the unknown dimensions of a target from the known dimensions of a standard target using electronic pointing and calculation tools that allow directly comparing on the video screen of a videoendoscope the image dimensions of the two above-mentioned targets. Thus, in patent U.S. Pat. No. 4,207,594, the diameter of the object field covered by the videoendoscope distal lens is used as standard target, the target to be measured being located in the object field. In a more realistic manner, documents GB 2 269 453 and IL 156 074 foresee using as a standard target a spot light with invariable dimensions obtained by projecting a collimated light beam on the area in which the target to be measured is located.
Measure by Projecting an Auxiliary Image on the Target
This method, used originally in traditional endoscopy, is for example described in patents DE 2 847 561 and U.S. Pat. No. 4,660,982. It consists in projecting on the target viewed by the endoscope a non-collimated image generated by a mask associated to a lens integrated into the distal end of the lighting device of the endoscope. The observation distance and the dimensions of the target can then be deduced from the position and dimensions, in the image field of the endoscope eyepiece, on the one hand from the image of the target, and on the other, from the image of the auxiliary image. This method is applied in videoendoscopy according to the implementation methods described in patents U.S. Pat. Nos 4,980,763 and 5,070,401. In parallel, the measurement using a similar method of a target located in the observation field of a video camera connected to a laser projector by applying to the target a non-collimated spot light has been described in patent FR 2 630 539. In these two methods, the use of electronic calculation and pointing tools on the video screen of the images of the target and of the auxiliary image has allowed simplifying the implementation of measurement procedures. Patent DE 3629435 describes another measurement method that can be used both in traditional endoscopy and videoendoscopy and combines a measurement method by comparison (use of a standard target formed by the images of two collimated laser beams parallel to the optical axis) and a measurement method by projecting an auxiliary image (image of a third collimated laser beam inclined with respect to the optical axis.)
Traditional Dual Optical Path Stereo Measure
This method consists in forming on the sensitive surface of the distal CCD sensor of a videoendoscopic probe two images of the target observed under different angles thanks to two distinct distal optical paths. The observation distance and dimensions of the target can therefore be deduced using the electronic pointing and calculation tools of the relative positions and the dimensions on the video screen of the probe of these two images. These two images can be simultaneously generated by two distinct lenses positioned at the distal end of the probe (U.S. Pat. No. 4,873,572, U.S. 2002/0137986, U.S. Pat. No. 6,063,023) or sequentially thanks to the alternating implementation of two pupils integrated into the distal lens of the probe and positioned symmetrically with respect to the optical axis (U.S. Pat. No. 5,222,477).
Measure by Image Splitting
There are also stereo display procedures implemented not in endoscopy, but rather in the stereoscopic television domain. These procedures originally consisted (for example according to patents U.S. Pat. No. 3,932,699, FR 2 619 664, FR 2 704 951, FR 2 704 951, FR 2 705 006) in associating two homothetic lenticular networks comprising the same number of image splitting unit optical elements, the smallest of the two networks was placed in front of the lens of a video camera, whilst the larger was adhered to the tube of a television receiver connected to said camera. A measurement method, derived from these display procedures is described in patent U.S. Pat. No. 4,993,380. The simultaneous implementation of LCD type video monitors composed of a pixel matrix and image processing programs that allows managing the signal received by each of these pixels has allowed improving these procedures, on the one hand by finely relating the structure of the reception lenticular network to that of the pixel matrix of the video monitor, and on other hand, by no longer using only one optical image splitting element at the video camera level, and thus improving the optical quality of said element. Thus the French inventor Pierre ALLIO, applicant of a large number of patents in this domain, has presented in the April 2003 issue of the French magazine “Science & vie” an image splitting optical device designed to be placed in front of the lens of a television camera and comprising:                a proximal optical element with a flat distal face and a proximal face equipped with a concave transversal profile,        a distal optical element with a distal face equipped with a convex transversal profile placed perpendicularly with respect to the concave profile of the proximal element and a proximal face in which two parallel and adjacent transversal strips are machined, each presenting an identical concave profile and placed perpendicularly with respect to the concave profile of the proximal element.        
As already indicated in patent DE 3 432 583, the implementation of a measuring procedure that uses a discrete image splitting device composed of a simple optical component with a flat distal surface and a projected edge delta-shaped proximal face has been described in patents U.S. Pat. No. 4,411,327 and U.S. 2002/0089583. Designed not to be integrated in a videoendoscopic probe, but rather to be placed in front of the lens of a video camera, this device is designed to measure the observation distance of a target placed in the vicinity of the optical axis of a traditional video camera.
It turns out that the stereo devices that simultaneously form on the CCD sensor two images of a target under two different view angles offers greater accuracy and repetitiveness of measurements. Nevertheless, the three stereo devices described above that allow obtaining such a result present significant, even redhibitory drawbacks as regards to their integration into a very small diameter videoendoscopic probe. Indeed, the integration of two axial view lenses in the axial end of a videoendoscopic probe requires the implementation of very small diameter lenses which prevent the entire sensitive surface of the CCD sensor connected to the lenses to be used. These limitations are not directed at the optimization of the accuracy of the measurement process. Furthermore, the integration into the distal end of a videoendoscopic probe of an image splitting device that have concave rectilinear transversal profiles becomes difficult to perform due to the miniaturization difficulties of such a device. In contrast, the implementation of a discrete image splitting device composed of a prism with a delta shape section does not present any problem in terms of miniaturization. Nonetheless, the optical measuring field of such a prism turns out to be, in principle, less than 15 degrees, a value that is clearly insufficient for an endoscopic measurement system.
The flexibility of use of a videoendoscopic probe supposes the possibility of adapting its distal end to removable and interchangeable observation heads that cover different field and/or view angle values. Obviously, it is preferable that the optical device necessary for implementing a measurement procedure can be also integrated into a removable head, knowing that in this case, the mechanical devices ensuring positioning and unlocking of the removable heads with respect to the CCD sensor of the probe must meet the highest accuracy requirements. In a general manner, the locking devices of a removable head on a distal end of a videoendoscopic probe must meet the following requirements:                continuity of the optical paths and light paths of the videoendoscopic probe and the removable head; this function requiring the simultaneous implementation of a longitudinal lock and a side indexation,        prevention of any possibility of accidental unlocking of the removable head,        absence of pollution of the CCD sensor due to emitted parasite light beams of the light path of the probe.        
The mechanical means implemented to meet these limitations vary according to the optical structure of the probe/removable head pair. More often, as is the case in patent U.S. Pat. No. 4,727,859, the CCD sensor of the probe is connected in a fixed manner to a distal optical device with a front compactness that is lower than the CCD sensor. The optical device can under these conditions be housed in the distal section of the probe that can therefore have a diameter that is less than the diameter of the probe in order to be able to be encased by the proximal tubular end of the removable heads. This architecture has the advantage of simplifying the locking devices and the disadvantage of having to house in the removable heads optical devices in additional to the one integrated permanently into the probe. On a purely optical point of view, it turns out to be more technically advantageous to house the entire optical system in removable heads that are fixed directly to the CCD sensor integrated into the probe, knowing that the mechanical locking devices will therefore be more delicate to design.